A new study reveals that headphones from digital music players may interfere with a pacemaker or an implanted defibrillator, as placing the headphones too close to the chest could interfere with the proper functioning of the devices that are meant to keep heart beats at their proper rhythm.

The results of the study were presented at the American Heart Association scientific meeting in New Orleans. One problem with the headphones is that they contain magnets, and “magnets may cause the pacemaker or implanted defibrillator to stop looking for abnormal heart rhythms,” Dr. William Maisel, the study’s lead investigator, said. Around 60 patients were used in order to conduct the research.

Researchers tested eight different models of MP3 player headphone with iPods. They found a detectable interference with the heart devices in 14 patients. In most cases, removal of the headphones restored normal device function and there were no problems when the headphones were at least 3cm from the skin’s surface. This confirms what U.S. health regulators reported last year about interactions between MP3 players and implanted heart devices that are unlikely to occur, if you keep the distance.

However, it’s the magnets that mess with the electrical impulses sent by a pacemaker, causing them to send beats no matter what. In defibrillators, the magnets could shutdown the device’s ability to send the electrical shock necessary to keep the heart beating normally.

The magnetic substance called neodymium is used in most MP3 headphones. Another part of the research proves that MP3 players themselves and cell phones with Bluetooth connection do not interfere with heart implanted devices. Dr. Maisel told the media: “For family members or friends of patients with implantable defibrillators, they should avoid wearing headphones and resting their head right on top of someone’s device.”